1. Field of the Invention
The present invention relates to an optical head position control device and particularly to an improvement in an optical head position control device in which focusing and tracking of an objective lens are effected by controlling a focus actuator and a tracking actuator on the basis of a detection signal of a light reception element which receives a laser beam reflected by an optical recording medium, the improvement being such that a sensor is provided for detecting the position of the objective lens and a negative feedback signal is superposed onto a servo signal for the focus actuator and the tracking actuator on the basis of the detection signal from the objective lens detecting sensor to thereby improve the operation margin for the focus actuator and the tracking actuator.
2. Description of Prior Art
As generally known, in an optical head, a laser is radiated onto an optical recording medium, such as a compact disk (hereinafter abbreviated as CD), so as to read out recorded information through the light reflected from the optical recording medium. The optical head is provided with a tracking mechanism for tracing a predetermined record track on the CD and a focusing mechanism for adjusting focus to cope with surface swing of the CD.
The tracking mechanism is provided with a radial drive mechanism for displacing the whole of the optical head by a motor drive and a tracking servo mechanism for finely displacing only the objective lens. The tracking servo mechanism is provided because the radial drive mechnism has a large inertia so that it can not move rapidly. The focusing mechanism is provided for verticaly displacing the objective lens so that the reflection plane of the CD is always within the depth of field, because the surface of the CD swings more or less due to warp thereof or the like. The optical head position control device according to the present invention is intended to control the objective lens position which is moved by the tracking servo mechanism and the focusing mechanism.
Referring to FIG. 1, a conventional optical head position control device is described. FIG. 1 is an explanatory diagram with respect to a focus servo in an conventional optical head position control device. In the drawing, reference numeral 1 designates a CD, 2 an optical head, 3 an objective lens, 4 a focus actuator, 5 a beam splitter, 6 a laser diode, 7 a PIN photodiode, 8 a focus error signal detecting circuit, 9 a phase correction circuit, and 10 an actuator drive circuit. As to the optical head 2, only the portions necessary for explaining the focus servo operation is briefly illustrated.
In FIG. 1, a laser beam emitted from the laser diode 6 is radiated onto the CD 1. A beam image reflected from CD 1 is received by the PIN photodiode 7 through the objective lens 3 and the beam splitter 5. Based on the output of the PIN photodiode 7, a focus error is detected by the focus error signal detecting circuit 8. The detection signal produced from the focus error signal detecting circuit 8 is then subject to phase correction in the phase correction circuit 9. Based on the correction signal produced by the phase correction circuit 9, the actuator drive circuit 10 controls the focus actuator 4 to adjust focusing of the objective lens 3. The tracking servo in the conventional optical head position control device is also performed in the same manner as described above as to the focus servo.
As generally known, the characteristic of an actuator is non-linear due to abrasion or play at a sliding portion thereof, influence of hysteresis, or the like. Further, variations in characteristic may occur in each actuator and variations in sensitivity and in resonance frequency may also exist among individual actuators. Accordingly, the conventional optical head position control device has a drawback that an increase in focus errors as well as tracking errors, deterioration in phase margin as well as gain margin, and variations in control characteristic may occur due to changes in temperature and humidity, or the like. Thus, conventionally, it has been intended to improve the characteristic of parts used and the structure of actuators to thereby improve the above-mentioned undesirable characteristics. Accordingly, the conventional device has a defect that the manufacturing cost is high.